lat/lon to time offset database

Chris Walton Chris.Walton at
Thu Feb 3 23:36:40 UTC 2011


Just so you know that it is possible... some of the newer GPS units on the market today are capable of automatically determining the local time zone.
This is achieved with proprietary time zone maps that are installed inside the GPS at the factory.
Newer versions of the maps are bundled inside firmware images so that you automatically get the latest map when you upgrade the firmware on your GPS.

If these built in time zone maps are not accurate (or if you find yourself in an area with disputed time zones), it is possible to turn off automatic mode and then manually enter the time zone of your choosing.
A Google search reveals that one of the GPS manufacturers stores its time zone data in a file that is approximately 600kB.
I think we can assume that the data in this file is written in a very heavily compressed format; it could theoretically be many megabytes if uncompressed.


From: Larry Ober [mailto:larry.ober at]
Sent: February 3, 2011 2:27 PM
To: tz at
Subject: Re: lat/lon to time offset database

Hi John,

Well there would be some level of accuracy question. The more accurate the polygon the larger the database. Also if the lat/lon data was provided by GPS, there is the question of signal availability.

Basically it just seems dumb that I need to tell my GPS what time zone it's in. LOL

Best Regards

----- Original Message -----
From: John Haxby<mailto:john.haxby at>
To: tz at<mailto:tz at>
Sent: Thursday, February 03, 2011 1:54 PM
Subject: Re: lat/lon to time offset database

On 3 Feb 2011, at 18:33, Larry Ober wrote:

I am looking at building a small embedded system (No OS) that will require
determining the local time from the lat/lon of a location. So basically I need
lat/lon to time offset from "GMT".

Does that work?   I'm sure I can find places in Spain that are that are close to Lisbon than they are Madrid but being in Spain the timezone should be Europe/Madrid.  Also, when you're on going through the channel tunnel do you go from being closer to Paris than you are to London only when you've left Calais?  Or before you get to Dover?

Or have I got hold of the wrong end of the stick?

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